Road surface purifies air by removing nitrogen oxides (NOx)

From the Eindhoven University of Technology. This is a neat idea that helps to solve a real pollution problem by automobiles. Cost of course is a factor, but considering some other schemes this may be a bargain. Now if they can just design some special cool asphalt for use around climate monitoring stations, we’ll really have something. – Anthony

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Road surfaces can make a big contribution to local air purity. This conclusion can be drawn from the first test results on a road surface of air purifying concrete. This material reduces the concentration of nitrogen oxides (NOx) by 25 to 45 per cent, said prof. Jos Brouwers in his inaugural lecture last Friday.

The tests were carried out in the municipality of Hengelo, where the busy Castorweg road was resurfaced last fall. As part of the project, around 1,000 square meters of the road’s surface were covered with air-purifying concrete paving stones. For comparison purposes, another area of 1.000 square meters was surfaced with normal paving stones.

Researchers at Eindhoven University of Technology (TU/e) carried out three air-purity measurements on the Castorweg last spring, at heights of between a half and one-and-a-half meters. Over the area paved with air-purifying concrete the NOx content was found to 25 to 45 per cent lower than that over the area paved with normal concrete. “The air-purifying properties of the new paving stones had already been shown in the laboratory, but these results now show that they also work outdoors”, said prof. Brouwers. Further measurements are planned later this year.

Asphalt

Brouwers, who has been professor of building materials in the TU/e Department of Architecture, Building and Planning since September 2009, sees numerous potential applications, especially at locations where the maximum permitted NOx concentrations are now exceeded. The concrete stones used in the tests are made by paving stone manufacturer Struyk Verwo Infra, and are already available for use. For roads where an asphalt surface is preferred the air-purifying concrete can be mixed with open asphalt, according to Brouwers. It can also be used in self-cleaning and air-purifying building walls.

The use of air-purifying concrete does not have a major impact on the cost of a road, Brouwers has calculated. Although the stones themselves are 50 per cent more expensive than normal concrete stones, the total road-building costs are only ten per cent higher.

Self-cleaning

Vehicle exhaust gases contain nitrogen oxides (NOx), which cause acid rain and smog. The air purifying concrete contains titanium dioxide, a photocatalytic material that removes the nitrogen oxides from the air and converts them with the aid of sunlight into harmless nitrate. The nitrate is then rinsed away by rain. These stones also have another advantage: they break down algae and dirt, so that they always stay clean.

Jos Brouwers, professor of building materials at Eindhoven University of Technology, delivered his inaugural lecture on the afternoon of Friday 2 July.

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48 thoughts on “Road surface purifies air by removing nitrogen oxides (NOx)

  1. Wow very cool stuff.
    As for coolness, if you had an asphalt of high silica content, it would absorb heat and hold it, reconducting it into the ground, like a space shuttle tile. Pumice gravel would be a suitable substitute. Its thermal conductivity is 50 times less than crystalline quartz, so it absorbs heat and holds onto it for a long time. This would moderate night temperatures around a surface station but would prevent high daytime highs that could result from excessive reflection or rapid re-radiation of heat by a surface.
    Ideally you would pack pumice gravel into a dense clay substrate which would give excellent load bearing and traction properties.

  2. The 2x cost of the NOx paving stones can likely be reduced through mass production. Interesting science project. Uses experiments and measures stuff – what an idea!

  3. Unless the surface can magically absorb CO2, not many will care. In Norway diesel powered cars have come very popular because the taxing system now favours them. The reason? Diesel cars emit less CO2, just about the only thing they emit less of. NOx, however, …

  4. Sounds fantastic, but I’m concerned on what happens to the nitrates. Do they have any figures on the quantity of nitrates that would be produced? If, as I suspect, it’s lower than the run off from garden/farm fertilisation, then I think this is a great idea! In particular, I’d like to see it used in tunnels, where air quality can be a problem.

  5. Best solution is dedicated bike trails. Second best is electric cars. It takes me just as long to drive to work as it takes me to ride, so I almost never drive – unless the roads are too icy.

  6. I’m not buying any of that.
    Automotive exhausts are hot, and therefore rise.
    Any consequential ‘absorption’ which might take place would be incidental.
    Oh, and another thing: NOx doesn’t cause so-called ‘acid rain.’ So that part of the story causes me no end of heartburn, and the intellectual ‘sniff test’ fails on that part for sure.

  7. OT, but amusing ad was displayed:
    Ads by Google
    Rajendra Pachauri -Nobel Peace Prize Winner at The London Speaker Bureau
    londonspeakerbureau.co.uk
    Perhaps not quite their target audience 🙂

  8. the total road-building costs are only ten per cent higher
    Well then, much cheaper than Steven Chu’s brainstorm:

  9. Surprised they didn’t mention that titanium dioxide is so pure snow white so it can raise the albedo too, combat some of the UHI.

  10. As a lifelong summer barefooter in California, I can assure you that you can get great local cooling by painting the asphalt under the thermometers with the white striping paint used on roads.
    Every single kid who was doing the Huck Fin thing through town would dash from paint stripe to paint stripe. You can simply walk or stand on the stripe, even when the black part is so hot you start doing the rain dance.
    I’d love to set up a Stevenson Screen and MMTS somewhere, over both asphalt and grass, and document the differential. Then you could paint the asphalt white and repeat the test…

  11. Amino Acids in Meteorites says: July 7, 2010 at 10:13 pm
    “the total road-building costs are only ten per cent higher”
    Well then, much cheaper than Steven Chu’s brainstorm:

    Painting the roses roofs white might have the same effect as the paving stones. Now that they’ve outlawed lead, titanium dioxide is the main pigment in white paint. See, you don’t have to be as bright as Hillary to score big. You just have to be lucky.

  12. stevengoddard says:
    July 7, 2010 at 9:56 pm
    “Best solution is dedicated bike trails.[…]”
    Unfortunately that potential has largely been depleted in Europe already so we will have to pay for this special “makes roads only 10% more expensive so it doesn’t really hurt, especially when we make the evil car owners pay for it by incrasing fuel tax” concrete. Can one step barefoot on that concrete on a sunny day without being absorbed catalitically?

  13. BTW, this study sounds a lot like the maker of the stones paid for it. You have a new product, you pay a research institute to do some tests on it and you or the people from the institute get the opportunity to exploit the measured data from the test in some papers.

  14. Do the three-way catalytic convertors that have been required on all U.S. autmobiles manufactured after 1981 somehow not work if they were required on cars in other countries?
    /sarc off

  15. As an alumnus from TU/e I am delighted to see a good idea put into practice, measurements being planned and executed, a reference being built into the experiment. It is likely that computer simulations were used in the design of the asphalt but using good engineering practices the performance has been confirmed through experiment. If only global warming science would adhere to these basics.
    Titanium dioxide dust (see: http://en.wikipedia.org/wiki/Titanium_dioxide#Health_and_safety) may be carcinogenic if inhaled. I would be interested to learn if the study has included possible the effects on health.

  16. “…titanium dioxide, a photocatalytic material that removes the nitrogen oxides from the air and converts them with the aid of sunlight into harmless nitrate. The nitrate is then rinsed away by rain.”
    So what about the impact on nitrate concentrations in surface water (>groundwater quality)? In recent years, many countries (all of the EU member states) have issued regulations to prevent nitrates from agricultural sources polluting ground and surface waters … So now that the farmers have invested in all kinds of techniques to get their nitrates down, let others go ahead and “pollute”.
    Yes I know, nitrogen oxdes are more harmful than nitrates, but still, I can imagine how those conscientious farmers will feel.

  17. Anthony,
    Having a rough time with this story.
    People actually tested this in a laboratory before testing again in the field?
    No computer model?
    Real measurements, with predicted values compared to actual?
    I guess there are still some real scientists out there.

  18. Amino Acids in Meteorites says:
    July 7, 2010 at 10:13 pm
    Thank you Amino. Very funny.
    I’ve been referring to wind turbine as bird grinders. Now I can start referring to Choo Choo.

  19. Painting asphalt roofs with titanium dioxide pigments may create other problems since asphalt shingles need to absorb UV light to avoid moisture under the shingles (rotting roofs) and to extend the shingle’s life since the heat helps keep the shingle from becoming too brittle and degrading. A metal roof might be the best overall since it can be coated, has a very long life and sound really cool when it rains. Maybe with enough nuclear power plants we could just melt all our old metal and make metal roof panels/shingles. One still needs energy and as cheaply as possible.

  20. I agree with Steven Goddard on this one: “Best solution is dedicated bike trails. Second best is electric cars.”
    How about if we spend the extra 10% on those two solutions (which will also help with other problems) rather than this one?

  21. They could build all roads so they only drive downhill. Save on fuel.
    As long as we have fear, we will see urgency in funding crazy notion research.

  22. Jack Simmons said:
    “People actually tested this in a laboratory before testing again in the field?
    No computer model?
    Real measurements, with predicted values compared to actual?
    I guess there are still some real scientists out there.”
    Would you consider Stephen Hawking to be a scientist?

  23. “Steinar Midtskogen says:
    July 7, 2010 at 9:48 pm
    Unless the surface can magically absorb CO2, not many will care. In Norway diesel powered cars have come very popular because the taxing system now favours them. The reason? Diesel cars emit less CO2, just about the only thing they emit less of. NOx, however, …”
    And PM10 particulates, very nasty stuff if not handled properly.

  24. Um, I’m no chemist… To convert NOx to nitrate, you have to react it with something
    else, don’t you? Is that “something else” in the concrete? If so the concrete will be
    corroded, wont it? Is the concrete porous? If so, it’ll be broken-up by ice in winter,
    won’t it?
    (EDIT: NOx is a monoxide. The titanium oxide photocatalyst causes NOx to react with free atmospheric oxygen to become nitrate, aka NO3, a trioxide. – The Mods)

  25. Ah, many thanks.
    I thought the ‘x’ was replaceable by any suitable number, so;
    NO, NO2, NO1/2 or N2O, NO3,…
    and that “nitrate” was a “salt” – a “base plus acid” of something,
    say copper oxide plus nitric acid -> copper nitrate.
    ?

  26. Why don’t they just get themselves a carbon air filter, plug it in with a long flex, & stand it outside? It will suck in all the dirty air, pump out lots of clean air, & when it’s full just change the carbon filter! It really is a wonder material. It filters out all sorts of nasties from the air & water! Many water companies use carbon filters on the filter beds when treating sewage.

  27. I hate to burst anyone’s bubble, but no one’s going to buy this unless it’s ‘Green’ — and I mean the actual color of the road surface. That’s what the world is looking for real green colored roads, concrete, steel, and glass. Back to the drawing board.
    PS: I know everyone goes to the Amazon Rain Forest to find new plants, etc., for everything under the Sun these days, but I just can’t keep thinking that the big bio-answer to the CO2-byproduct construction problem is to be found in the Sahara; you know, some little thing that grows on rocks and sand and uses CO2 & H2O and gives off O2?

  28. Good question there from Dave Springer:
    July 8, 2010 at 12:26 am

    Do the three-way catalytic convertors that have been required on all U.S. autmobiles manufactured after 1981 somehow not work if they were required on cars in other countries?
    /sarc off

    Euro 5 or Euro 6 vehicle emission standards apply to all new vehicles in Europe. Euro 6 still allows slightly higher NOx from compression ignition engines than from spark ignition; 80 vs 60 mg/km. Think about how little that is. The Euro 5 to Euro 6 reduction for diesel engines is from 180 down to 80. More than a halving.
    Comression ignitions tend to produce more NOx because they operate very lean most of the time. There is a lot of “excess air” which is good for combustion efficiency, but the high temperatures and the majority of air being N2 results in the formation of NOx. A 3-way catalyst cannot convert the NOx to N2, etc under those conditions. It needs unburnt fuel, HC which would mean a large increase in fuel consumption for the same engine output.
    NOx formation can be reduced without large penalties in engine efficiency by means of cooled, exhaust gas recirculation (EGR). Exhaust gases contain less “free'” O2 for combustion, even in a lean-burning diesel. The “inert” gas also acts to dampen the temperature rise during the combustion process; a heat sink. That’s a small penalty in efficiency but much, much less than trying to produce HC by over-fuelling!
    In the exhaust post-treatment, which now is almost always combined with cooled EGR. For Euro-4, it was sufficient to have a NOx storage “catalyst” on smaller diesel engines. Whenever the storage is near “full”, the outlet’s NOx level rises. Engine management programs running “rich” for a little while, adjusting injection timing so that the driver feels no difference in vehicle performance. That is of course to the detriment of fuel consumption and for about the same penalty in fuel consumption, it’s much more rewarding for the driver to occassionally “make smoke” by hard acceleration when conditions allow. 🙂
    Euro-5/6 limits are more strict and other technology is called upon. Urea (or ammonia) injection is one technology. This results in a “selective, non-catalyst reduction (SNCR)” of the oxides of nitrogen. It requires only a few litres of the stuff every 10Mm or so per litre of engine size.
    Traffic conditions tend to dictate where certain type of emissions occur. Engines running lean due to a backing-off of drivers on approach to an obvious hazard will tend to produce more pre-catalyst NOx. Those areas where engine power is called upon, like on-ramps to motorways will tend to seen more pre-catalyst HC.
    With a little knowledge, it’s not hard to design an “experiment” which produces the desired result.

  29. Steven Goddard
    I bike too but where do you put a bike trail into a city-through my back yard? Biking in traffic is also bad for your lungs. Also, the electric car batteries require lots of energy to produce and lots of pollution in their construction. I believe that hydrogen cars are really the way to go. Several problems remain to overcome in both electric and hydrogen types . Too bad more of this windmill money was not being spent on both types.
    You must be alot tougher than I am to be riding in that Colorado weather-especially this year!

  30. Sleepalot says:
    July 8, 2010 at 6:34 am
    Ah, many thanks.
    I thought the ‘x’ was replaceable by any suitable number, so;
    NO, NO2, NO1/2 or N2O, NO3,…

    NOx in the atmosphere refers to NO + NO2, they’re constantly being converted to each other but ∑(NO,NO2) remains fairly constant so it’s convenient to treat them as a single compound (NOx). In the atmosphere the NO2 will react with H2O to form Nitric acid and hence ‘acid rain’. The problem with NOx in cities due to heavy traffic is that in summer is that photocatalysed reactions lead to local formation of ozone and photochemical smog. The surface reaction of NO2 on TiO2 is also photocatalysed so you’d expect it to work when it’s most needed, works better with some water.
    and that “nitrate” was a “salt” – a “base plus acid” of something,
    say copper oxide plus nitric acid -> copper nitrate.
    ?

  31. Reminds me of back in the day
    I worked years ago with Englehardt on catalytic car radiators. We were going to plate car radiators with a catalyst, and as people drove around they would purify the air.
    Even had a mini fleet of cars driving around with all the instrumentation and so forth.
    Everything was looking good until some smart ass did the math on how many cubic meters of air were in the Los Angeles basin and how many radiators/sq meters/hours we would need to make a difference.
    And we found there were not enough cars in Southern California. We needed more cars and people had to drive more. So that idea went on the “not sellable” spike.
    Actually it now sounds like another policy initiative from our leaders. “We need people to drive more so we can have cleaner air”

  32. Come on!, they are just trying to sell us their titanium oxides they produce a lot. First try was when someone wanted to paint all roofs white.
    The best NOx absorber is Urea (carbo di amide, (NH2)2CO ), it reacts with NOx to form ammonium nitrate, a fertilizer. Some german cars already have such a catalyzer.
    BTW Who´s in the business of titanium white pigments?…You know:”Follow the money”….

  33. I just got a better idea!!!!.
    What do BEDWETTERS produce the most?…..PEEE, that´s it: UREA
    just attach a convenient container connected to engine exhaust and fill it with bedwetters´pee!!

  34. Paving stones? As a well experienced highway engineer, I shudder to think of the future maintenence costs. If there ever was a step backwards in highway engineering this is it. I do wonder when they will rediscover cobblestones. Or roads metaled with aggregate. Artificial (manufactured) white stone (gravel) roads, it would seem, would work quite well indeed. And what fun to drive on.

  35. One concept that could also be used to reduce pollution in cities would be to have hybrid cars which can run only electric whenever you get out of the highway or big boulevard. Same thing could be done with helicopters and planes, they would takeoff and land on electricity. So less pollution and noise where people live with the commodity of oil powered engines.

  36. @Larryoldtimer: One of the problems you have when more than half of your country is build on peat and such: your roads tend to sink into the underground. So almost every town in the Netherlands uses paving stones. Those you can pick up, put a fresh layer of sand underneat, and resurface the road using the old stones. You can see for yourself: go to Google Earth, and look up any town in the Netherlands using StreetView. You’ll be surprised…
    @StevenGoddard: never been to the Netherlands? the only place on earth were they have more bicycles than inhabitants, and were just everybody cycles to work. Don’t believe that? Again go to Google Streetview, and select any town with a railway station. Look around the station. People are up to their armpits in bikes.
    And while you’re at it: look at the color of the pavement. Any road surface painted reddish-purple is a bike-only lane. no cars allowed. And even better: cyclists are protected by law. If you hit a cyclist with a car it is up to you to prove that said cyclist was in the wrong. You are not going to do that, except when the cyclist is DUI. Otherwise you are going to pay for all damages.
    Pjotr

  37. Just imagine a country where ALL THE IDEAS OF THE GREENS could have been implemented, this is an theme for a comediant…..just imagine ☺☺☺☺☺

  38. I would have thought we already had this because a few years ago I’d heard about structure (aka building) concreate that could do this. Maybe it needed a different psi rating for the road. Standard house fondation is only 3000lb, airports use upwards of 9000lb. I don’t know what roads would use. Immagine using not just roads but building foundations for this, With wide spread use of a NOx scrubing concreate, we could get rid of most of it.

  39. Marc77 says:
    July 8, 2010 at 11:21 am
    One concept that could also be used to reduce pollution in cities would be to have hybrid cars which can run only electric whenever you get out of the highway or big boulevard. Same thing could be done with helicopters and planes, they would takeoff and land on electricity. So less pollution and noise where people live with the commodity of oil powered engines.
    You’ve really no real knowledge of –or experience in– aviation, have you?
    Would you care to guess the size of the battery –not to mention the electric motor– would be for just a helicopter?
    Not only the size but the WEIGHT!
    The essence of your thinking would result in a monstrosity of such dimensions that it would be not unlike a steam locomotive with wings!!
    Care to guess how far that would fly?!?!?!

  40. Daniel says:
    July 8, 2010 at 5:23 am

    Would you consider Stephen Hawking to be a scientist?

    Yes.

  41. Jack Simmons says:
    July 8, 2010 at 6:00 pm
    Daniel says:
    July 8, 2010 at 5:23 am
    Would you consider Stephen Hawking to be a scientist?
    ………………………………………………………………………………………………………………..
    Depends.

  42. Having worked at Hengelo in a Joint Venture with Akzo on titanium dioxide in 1974-6 and having the pleasure to know Dick Thoenes who went from there to be a Professor Emeritus in Chemical Engineering at Eindhoven, my bet would be that the chemistry is sound and the principles are good. Please do not knee-jerk on this one.

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